Electromechanical elastic wave delay line

ABSTRACT

An electromechanical elastic wave delay line useful, for example, in television signal processing circuitry, has an input transducer coupled to an output transducer by a delay body capable of propagating torsional elastic waves. The transducers each comprise a washer-like annular disc of ferroelectric material, diametrically opposed portions of which are electrically poled in opposing directions. Diametrically opposed electrodes are driven in push-pull to generate torsional elastic waves in the coupling body. The delay body is capable of transmitting torsional elastic waves at relatively high frequencies, for example 4 - 5 megahertz or higher. The delay body is shown in the form of a cylindrical tube having a section of reduced radial thickness for establishing a maximized cut-off frequency below which torsional elastic waves in non-zero order modes are incapable of being supported in the body. In another embodiment the body is shown as a rod having a necked down section for establishing the cut-off frequency of the body. Means for damping reflected waves and a method for adjusting the delay in the line are also disclosed.

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' x We '7] ELECTROMECIIANICAL ELASTIC WAVE DELAY LINE [75 Inventor: Roger W. Knitter, Hoffman Estates,

[73] Assignee: Zenith Radio Corporation, Chicago,

Ill.

[22] Filed: Dec. 3, 1971 [21] Appl. No.: 204,482

[52] US. Cl 333/30 R, 333/71, 333/72, 310/85 [51] Int. Cl H03h 9/26, H03h 9/30, H04b 11/00 [58] Field of Search 333/30 R, 72, 71, 333/98 M; 315/83, 8.5, 8.6

[56] References Cited UNITED STATES PATENTS 3,252,336 5/1966 Eisner 3lO/8.5 X 3,252,335 5/1966 Eisner 310/85 X 2,573,168 10/1951 Mason et a1. 310/8.l 3,505,657 4/1970 Whitehouse 333/30 R X 3,177,450 4/1965 Tzannes-et al.... 333/71 X 3,537,039 10/1970 Schafft 333/30 R 3,262,769 7/1966 Brauer.... 333/30 X 3,098,204 7/1963 Brauer 333/30 R 2,667,621 1/1954 Burns, Jr. et al. 333/71 FOREIGN PATENTS OR APPLICATIONS 758,647 10/1956 Great Britain 333/30 M OTHER PUBLICATIONS Beardow, Waveguide Manufacturing Techniques" in British Communications and Electronics October 1958, pp. 772, 774.

CROSS REFERENCE 1 [11} 3,757,257 Sept. 4, 1973 Mason, Physical Acoustics" Vol. I Part A, Academic Press, New York. 1964, Title Page and pp. 418-420.

Primary ExaminerRudolph V. Rolinec Assistant Examiner-Marvin Nussbaum Attorney-John H. Coult et al.

[5 7] ABSTRACT An electromechanical elastic wave delay line useful, for example, in television signal processing circuitry, has an input transducer coupled to an output trans ducer by a delay body capable of propagating torsional elastic waves. The transducers each comprise a washerlike annular disc of ferroelectric material, diametrically opposed portions of which are electrically poled in opposing directions. Diametrically opposed electrodes are driven in push-pull to generate torsional elastic waves in the coupling body. The delay body is capable of transmitting torsional elastic waves at relatively high frequencies, for example 4 5 megahertz or higher. The delay body is shown in the form of a cylindrical tube having a section of reduced radial thickness for establishing a maximized cut-off frequency below which torsional elastic waves in non-zero order modes are incapable of being supported in the body. In another embodiment the body is shown as a rod having a necked down section for establishing the cut-off frequency of the body. Means for damping reflected waves and a method for adjusting the delay in the line are also disclosed.

7 Claims, 7 Drawing Figures CROSS-REFERENCE TO RELATED APPLICATION This application is related to, but not dependent upon, copending application of R. Adler, Ser. No. 204,484 filed Dec. 3, 1971, now US. Pat. No. 3,719,907 dated Mar. 6, I973 assigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION The present invention relates generally to elastic wave transmission systems, and is particularly directed to torsional elastic wave delay lines for use in electronic systems such as television signal processing circuitry wherein it is desired to delay one electrical signal relative to another.

The potential advantages of electromechanical delay lines over conventional electrical delay lines have long been recognized. However, practical problems of con struction and the relatively high costs involved have confined their use to the laboratory only.

OBJECTS OF THE INVENTION It is a general object of this invention to provide improved electromechanical elastic wave delay lines.

It is another general object to provide improved elastic wave propagative bodies for use in elastic wave transmission systems of general character.

' It is a less general object to provide elastic wave delay lines producing lower dispersion than prior electromechanical elastic wave delay lines.

It is yet another object to provide an electromechanical delay line of the nature described which may be accurately adjusted to achieve a desired time delay.

It is a further object to provide improved electromechanical delay lines having provision for damping reflected elastic waves.

It is a further object to provide low dispersion torsional wave delay lines which are practical and rela tively inexpensive to manufacture.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several figures of which like reference numerals identify like elements.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side-elevational view, partially schematic and partially in cross-section, of a delay line embodying the present invention;

FIG. 2 is a view of one end face of a torsional-mode transducer employed in the delay line of FIG. 1;

FIG. 3 is a transverse cross-sectional view taken along line 3 3 of FIG. 2;

FIG. 4 is a view of the opposite end face of the transducer of FIG. 2;

FIG. 5 is a diagram showing one method of poling a ferro-electric disc for use in the transducer of FIG. 2;

FIG. 6 is a cross-sectional view taken along line 66 of FIG. 5; and

FIG. 7 is a fragmentary schematic side elevational view of an alternative embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a signal source 2 drives an input transducer 3 in push-pull. Transducer 3 induces torsional elastic waves in wave-propagative delay body 4. Subsequently, the waves generated by transducer 3 interact with an output transducer 5 to effect the development of delayed electrical signals that are fed in push-pull to a load 6.

As specifically illustrated, FIGS. 2-4 depict details of input transducer 3 and FIGS. 5 and 6 pertain to a step in its formation. It is to be understood, however, that all of FIGS. 2 6 apply as well to output transducer 5. Thus, transducer 3 includes an annular disc 10 of a ferro-electric material such as PZT (lead zirconate titanate). Disc 10 is of generally washer-like configuration, having a centrally located bore 12 and provided with flat parallel end faces 14 and 16. The thickness of disc 10 is chosen so as to produce a shear wave resonance corresponding to an impedance minimum across the electrodes, at the nominal or center frequency of the signal from source 2. A unitary electrode 18, preferably a metallic film of gold or silver, is bonded to completely cover end face 16 of disc 10. A pair of driving electrodes 20a and 20b, which likewise may be a film of gold or silver, are bonded to the opposite end face 14 of disc 10 in spaced symmetrically disposed locations on opposite sides of an imaginary diametrical line on face 14 that would be along line 3 3.

Prior to its assembly into the completed transducer of FIGS. 2 4, disc 10 is electrically poled by the arrangement shown in FIGS. 5 and 6. Poling electrodes 22a and 22b are in contact with the disc on diametrically opposed sectors. Each of electrodes 22:: and 22b is of relatively small circumferential extent and, as indi' cated in the cross-sectional view of FIG. 6, extends around both end faces of disc 10 and across both the inner and outer cylindrical surfaces of the disc. By using comparatively narrow poling electrodes, a maximum of the ferro-electric material is utilized and the magnitude of strains produced in the material during poling is reduced. Advantageously, the width and shape of the poling electrodes are the same as those of the spaces between driving electrodes 20a and 20b.

Poling is accomplished by applying a relatively high DC voltage (e.g., 75,000 volts per inch) across electrodes 22a and 22b to induce circumferential polarization in disc 10 extending from sector 22a to sector 2212 around the disc axis. Disc 10 is poled in mutually opposing directions in diametrically opposing halves of the disc. As indicated by the arrows P in FIG. 5, a major portion of each half of the disc is poled and the poling direction is clockwise in one half of the disc and counterclockwise in the other half. It will be recognized that each of the portions poled in one specific orientation corresponds to the position of one of the two electrodes 20a and 20b in the completed transducer. After poling of disc 10 has been completed, poling electrodes 22a and 22b are removed from the disc by etching or the like and electrodes 18, 20a and 2% then are applied in registry with the poled portions of the disc. When electrodes 20a and 20b are used in a push-pull circuit as shown, the oppositely directed axial electric signal fields generated or picked up by these electrodes, in cooperation with the correspondingly Opposed directions of polarization, couple to purely torsional strain in disc 10.

In principle, bore 12 may be omitted. During poling, however, this leads to the development of very high voltage stress in the center of disc as a result of which arcing may occur between the poling electrodes. Even with bore 12, it is preferred during poling to immerse disc 10 in a highly refined oil or other highdielectric liquid medium.

Returning to FIG. 1, delay body 4 takes the form of an elongate hollow cylindrical metal tube of a suitable material, such as stainless steel or N i-Span-C alloy 902, capable of propagating torsional elastic waves. The delay body 4 will be discussed in detail below in connection with a description of the present invention.

A transducer of the type shown in FIGS. 2 4 is bonded to each end of the tube by its unitary or solid electrode 18. An AC signal from source 2 is applied in push-pull to electrodes a and 20b of the transducer at the left-hand end of the tube as viewed in FIG. 1; that signal is transduced by transducer 3 into a torsional elastic wave which is applied to the left-hand end of rod 24. The torsional waves thus induced travel through the delay body 4 from left to right, inducing a corresponding torsional elastic wave in transducer 5 at the righthand end of delay body 4. Transducer 5 converts that torsional strain into an electrical signal which is applied in push-pull to load 6.

In practice, for the production of a delay line operative at a nominal or center frequency of 3.58 megahertz for use in a color-television receiver, for example, typical parameters of the transducers may be: PZT having a low-frequency dielectric constant greater than 450, an effective shear coupling factor greater than 55 percent, surface parallelism better than 0.0001 inch, outside diameter of 0.256 inch, inside diameter of 0.180 inch and thickness of 0.016 inch. The latter, of course, is most critical; whatever the piezoelectric material used, the thickness should be chosen to produce a resonance, corresponding to an impedance minimum across the driving electrodes, at the frequency of interest.

In use, it has been found that delay lines incorporating the transducers herein disclosed may be designed for a wide range of frequencies. The resulting signal field is axial, and the interposed unitary electrode provides complete electrical separation from the associated transmission line. Moreover, the delay line body as well as the individual transducers are capable of economic mass production.

From well known theory, a cylindrical rod delay medium capable of supporting a torsional elastic wave in the fundamental mode only (the presence of other modes would introduce dispersion) at 5.0 megahertz, for example, will have a diameter of only 0.160 inch with a wall thickness of 0.010 inches. Rods of this size are difficult to work with and quite susceptible to damage.

The more severe problem, however, as may be appreciated from the above discussion, centers on the difficulties encountered in attempting to fabricate and attach transducers having dimensions compatible with the end faces of such rods,

According to one aspect of this invention, elastic wave delay bodies are provided which have the necessary small dimensions transverse to the direction of wave propagation to allow high frequency applications,

and yet which present relatively large end dimensions to facilitate coupling' to input and output transducers, which may, for example, be transducers of the nature described above.

FIG. 1 illustrates a preferred implementation of this concept in which a delay medium is shown in the form of a cylindrical tube. The tube cross-sectional dimensions, specifically its wall thickness and to a lesser degree its mean diameter, determine the frequency limit (or cut-off frequency f below which torsional elastic waves may be supported in the fundamental mode only. By the use of a tube, rather than a rod, as a wave propagative delay medium, an end area is provided which is large enough to be readily coupled to available high performance transducers of the nature described above.

Using the combination of transducers and delay medium suggested above, low dispersion electromechanical delay lines having operating frequencies in the low megahertz range have been constructed and tested with satisfactory results. However, the construction of an electromechanical delay line which is useful, for example, at 4 5 megahertz, even using the novel combination above described, calls for a wall thickness on the tubular delay body in the order of 0.010 to 0.012 inch. It has been found that wave delay tubes having such thin walls are difficult to mate with transducers and result in relatively low transducer efficiencies.

According to one aspect of this invention, viewed in broad sense, there is provided for use in an elastic wave transmission system an elastic wave propagating body having a transverse cross-section which varies in its dimensions along the length of the body, i.e., along the direction of propagation of the elastic waves, such that at different predetermined points along the length of the body, the body is capable of supporting elastic waves in the fundamental mode only at frequencies up to different maximum (cut-off) frequencies.

This concept is employed to minimize the described transducer mating problems. Referring to FIG. 1, by this invention there is provided a wave propagating body including a first section 30 having preselected transverse dimensions (mean diameter and radial wall thickness) determining a cut-off frequency below which waves can propagate only in the fundamental (zero order) mode.

A coupling section 32 between the first section 30 and the elastic wave generating means (transducer 3 in the FIG. 1 embodiment) is shown as having preselected transverse dimensions (mean diameter and wall thickness) which are greater than the dimensions of the first section 30, thereby facilitating connection of the delay body 4 to transducer 3.

Because the coupling section 32 has greater transverse dimensions than the section 30, section 32 has a lower cut-off frequency than the section 30. For purposes of discussion, assume the cut-off frequency (again, the frequency below which a body may propagate torsional elastic waves in the fundamental mode only) for section 32 to be f, and the cut-off frequency for section 30 to be f,, where f, is less than f,. Sections 30 and 32 will both support waves in the fundamental mode only, and no higher order modes, in the frequency range from D.C. to f,. In the frequency range f to f,, higher (non-zero) order modes will be supported in coupling section 32. However, section 30 will not support torsional elastic waves in higher order (non-zero) modes at frequencies below f Therefore, waves in the higher order modes in section 32 in the frequency range f to f, will be very severely attenuated in section 30. As an example, consider a tubular delay element composed of a material such as Nickel-Span-C alloy 902, for example, and having a section 32 with a wall thickness 0.012 inch and a section 30 with wall thickness 0.0l inch. If the outside diameter of this tubing is 0.160 inch and 0.156 inch in sections 32, 30 respectively, then the cut-off frequencies f, and f; are approximately 4 and 5 megahertz respectively. The cut-off frequencies change if material constants of the delay body change.

A length may be readily selected for section 30 which is effective to substantially completely attenuate such higher order modes in the frequency range f, to f For example, a section 30 of the described embodiment one inch in length will attenuate the first order mode of a 4.8 megahertz signal by more than 100 db. Nonzero order modes are highly dispersive and the effectiveness with which these modes are suppressed and the magnitude of the cut-off frequency which can be achieved, to a large measure determine the practicality of electromechanical delay lines.

Thus, from one perspective this invention may be viewed as providing means by which a propagative body having a relatively high cut-off frequency can be coupled to real-world transducer structures. Viewed from another perspective this invention may be thought .of as teaching a way to attenuate non-zero order modes supported in a first portion of a delay medium by providing an attenuating portion cascaded with the first portion which is incapable of supporting such non-zero order modes.

In order that the section 30 may be coupled to an output transducer such as transducer 5 in FIG. 1, and to preserve the symmetry of the delay line, a second coupling section 34, preferably constructed like coupling section 32, is provided between section 30 and transducer 5.

To minimize reflections from the boundaries between section 30 and coupling sections 32 and 34, in accordance with another aspect of this invention, there are preferably provided transition sections 36 and 38. In the illustrated embodiment, the transition sections 36, 38 are each shown as tapering from a radial wall thickness equal to the wall thickness of the mating ends of the associated coupling sections 32, 34 down to a lesser wall thickness corresponding to the wall thickness of section 30.

In a broader sense, the transition sections 36, 38 represent an implementation of a teaching of this invention that a wave propagative delay body may be provided for use in an elastic wave transmission system, which body has the capability of supporting elastic waves in the fundamental mode everywhere but which hasdifferent cut-off frequencies for higher order modes at different points along its length. The illustrated transition sections 36, 38 exhibit a linear taper and would therefore have an approximately linear cutoff frequency-versus-length characteristics.

As is well known, even assuming optimized impedance matching at each end of the delaybody 4, wave reflections can be suppressed at only one, or at most, a very limited number, of wave frequencies. Thus, in a wave transmission system such as an electromechanical delay line for use in television processing circuitry, having a relatively wide bandwidth, substantial wave reflections will be produced in the delay body 4.

In accordance with yet another aspect of this invention there is provided means for damping torsional elastic waves propagating in the delay body, and more particularly, means for strongly clamping reflected waves. Referring to FIG. 1, reflected wave damping means, here shown as strips 40, 42 of adhesive tape, are disposed in firm and intimate engagement with the outer surface of at least a portion of the delay body 4 for damping torsional elastic waves traversing the portion or portions of the body on which the strips 40, 42 are disposed. Because reflected waves traverse the portions of the body 4 carrying the damping strips 40, 42 a number of times, depending on the number of iterations of the wave, reflected waves are strongly suppressed.

By this expedient, spurious signals resulting from waves reflected in the delay line are substantially reduced. It is contemplated that means other than adhesive tape may be employed to damp the torsional strains produced in the propagative medium. It is preferred that where the damping means presents an edge in its interface with the propagative body, that the edge be oblique to the direction of wave propagation. In FIG. 1, for example, the edges 44, 46 of strips 40, 42 are slanted with respect to the axis of the body 4 (also the direction of wave propagation).

Prior art electromechanical delay lines have been so constructed that it was difficult to adjust them accurately to a predetermined delay. By this invention there is provided a method of adjusting the effective delay in an elastic wave delay line which includes a deformable elastic wave propagating body. The method involves forming the propagative body from a deformable material such as certain metals, which are capable of propagating elastic waves and which are also capable of being stretched. Ni-Span-C alloy 902 is a satisfactory material. The body is cut or otherwise formed to have a gross length corresponding to a delay which is less, preferably only slightly less, than the desired delay. The body is then stretched with permanent deformation until it attains a length corresponding to the desired delay time. The delay time of the body may be continuously or periodically monitored during the stretching process. Alternatively, the body may be cut to a toolong gross length and compressively deformed to the appropriate length.

This invention is not limited to the particular details of construction of the embodiments depicted, and it is contemplated that various other modifications and applications will occur to those skilled in the art. As examples: 1) other materials than those suggested for the delay body may be employed; 2) other constructions and shapes for the delay body may be employed, depending upon the overall cut-off frequency desired, upon the desired attenuation of non-zero order modes supported in a part of the body or generated by the wave generating means, upon the size and configuration of the transducers to be coupled to the delay body, and upon other factors; 3) other wave generating means than those disclosed may be employed to feed wave-propagative bodies following the teachings of this invention; 4) the teachings of this invention may be employed in elastic wave transmission systems in general and are not limited to employmentin electromechanical delay lines; 5) reflected wave damping means other than those shown are within the purview of this invention; 6) symmetry in the delay body or associated system is not required.

As a further example, FIG. 7 depicts certain of the teachings of this invention incorporated in an electromechanical delay line having transducers 48, 50 coupled to a solid rod delay body 52. The body 52 has coupling sections 54, 56 joined by transition sections 58, 60 to a section 62 which may be thought of as the cutoff frequency-determining section, or alternatively, as an attenuating section for blocking the transmission of waves in the dispersive non-zero order modes at frequencies below the cut-off frequency. The functions and actions of the coupling sections 54, 56, the transition sections 58, 60 and the cut-off frequencydetermining section 62 in the FIG. 7 delay line are analogous to the functions and actions of the corresponding sections in the H08. 1-6 delay line.

It is evident therefore that certain changes may be made in the above-described apparatus without departing from the true spirit and scope of the invention herein involved. and it is intended that the subject matter of the above depiction be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. A longitudinally symmetrical cylindrical hollow torsional elastic wave propagating body having a crosssection transverse to the direction of wave propagation which is smaller in diameter in a center section than at the end sections of such body and including tapered transitional sections between said center section and said end sections for minimizing wave reflections such that said body is capable of supporting torsional elastic waves in a fundamental mode exclusively at frequencies up to the maximum frequency supportable by said center section.

2. An electromechanical information transmission system comprising:

a hollow cylindrical body capable of propagating elastic waves, said body having preselected dimensions transverse to the direction of wave propagation determining a first frequency limit below which waves can propagate only in a fundamental mode;

wave-generating means for generating elastic waves in said body;

means sensing said elastic waves in said body;

a first hollow cylindrical coupling section between said wave-generating means and said body for coupling waves generated by said wave-generating means into said body;

a second hollow cylindrical coupling section between said sensing means and said body, said first and second coupling sections having preselected dimensions transverse to the direction of wave propagation greater than said dimensions of said body such that said coupling sections have a second frequency limit for wave propagation exclusively in a fundamental mode which is lower than said first frequency limit, said body serving to attenuate elastic waves in non-zero order modes at frequencies below said first frequency limit of said body which might be generated in said wave generating means or supported in said coupling sections; and

hollow cylindrical transitional sections for smoothly tapering said coupling sections to said body to minimize reflections therein.

3. The system defined by claim 2 wherein said elastic waves are torsional waves, and wherein said body has a smaller radial thickness than said coupling sections.

4. An electromechanical delay line, including:

an elongate hollow cylindrical body capable of propagating torsional elastic waves, said body having a length corresponding to a predetermined torsional elastic wave propagation time;

transducing means coupled to one end of said body for receiving an input electrical signal and for launching torsional elastic waves in said body carrying information corresponding to information carried by said input electrical signal;

transducing means coupled to the opposite end of said body for sensing said information bearing elastic waves and converting them to an output electrical signal;

said body having an attenuating section of radial thickness which is less than the radial thickness of other sections of said body for attenuating torsional elastic waves in non-zero order modes at frequencies below a preselected maximum frequency determined by the dimensions and configuration of said attenuating section, each of said transducing means comprising;

a disc of ferro-electric material of a thickness corresponding to shear wave resonance at a predetermined operating frequency, having flat parallel end faces with a major portion of each half of said disc being electrically poled circumferentially in mutually opposing directions in diametrically opposing portions of the disc;

a pair of electrodes bonded to one end face of said disc in registry with said major portions of said disc halves; and

a unitary electrode bonded to and overlying the other end face of said disc.

5. A torsional-wave delay line comprising:

an elongate hollow cylindrical tube propagative of torsional-mode waves;

a pair of circular discs, each having a thickness corresponding to shear wave resonance at a predetermined frequency, of ferro-electrical material with each being electrically poled circumferentially in mutually opposing directions in diametrically opposed halves of the disc;

first and second electrode pairs individually bonded to respective one end faces of said discs with the electrodes of each pair being disposed in registry with said diametrically opposing halves of said discs;

first and second unitary electrodes individually bonded to and overlying the respective other end faces of said discs; and

means for effectively bonding said discs individually to the respective opposite ends of said tube with said unitary electrodes individually engaging the respective ends of said tube.

6. A torsional-wave delay line comprising:

an elongated hollow cylindrical tube propagative of torsional elastic waves, said tube having a generally uniform cross-sectional configuration and radial wall thickness along a first section of its length and a second section having substantially uniform cross-sectional configuration and a radial thickness which is less than the radial thickness of said first section;

a pair of circular discs, each having a thickness correbonded to and overlying the respective other end spending to shear wave resonance at a prdeterfaces of s id discs; and mined frequency, of ferm'electric material with means for effectively bonding said discs individually each being electrically poled circumferentially in mutually opposing directions in diametrically op- 5 posing halves of the disc;

to the respective opposite ends of said tube with said unitary electrodes individually engaging the first and second electrode pairs individually bonded 7 respelcnv? ends of smd to respective one end faces of said discus with the A de ay hne as defined m claim 6 which further electrodes of each pair symmetrically Spaced on cludes means for applying an electric signal in pushopposite id of an imaginary di i Heron l0 pull across the individual electrodes of one of said their one end face; pairs.

first and second unitary electrodes individually 

1. A longitudinally symmetrical cylindrical hollow torsional elastic wave propagating body having a cross-section transverse to the direction of wave propagation which is smaller in diameter in a center section than at the end sections of such body and including tapered transitional sections between said center section and said end sections for minimizing wave reflections such that said body is capable of supporting torsional elastic waves in a fundamental mode exclusively at frequencies up to the maximum frequency supportable by said center section.
 2. An electromechanical information transmission system comprising: a hollow cylindrical body capable of propagating elastic waves, said body having preselected dimensions transverse to the direction of wave propagation determining a first frequency limit below which waves can propagate only in a fundamental mode; wave-generating means for generating elastic waves in said body; means sensing said elastic waves in said body; a first hollow cylindrical coupling section between said wave-generating means and said body for coupling waves generated by said wave-generating means into said body; a second hollow cylindrical coupling section between said sensing means and said body, said first and second coupling sections having preselected dimensions transverse to the direction of wave propagation greater than said dimensions of said body such that said coupling sections have a second frequency limit for wave propagation exclusively in a fundamental mode which is lower than said first frequency limit, said body serving to attenuate elastic waves in non-zero order modes at frequencies below said first frequency limit of said body which might be generated in said wave generating means or supported in said coupling sections; and hollow cylindrical transitional sections for smoothly tapering said coupling sections to said body to minimize reflections therein.
 3. The system defined by claim 2 wherein said elastic waves are torsional waves, and wherein said body has a smaller radial thickness than said coupling sections.
 4. An electromechanical delay line, including: an elongate hollow cylindrical body capable of propagating torsional elastic waves, said body having a length corresponding to a predetermined torsional elastic wave propagation time; transducing means coupled to one end of said body for receiving an input electrical signal and for launching torsional elastic waves in said body carrying information corresponding to information carried by said input electrical signal; transducing means coupled to the opposite end of said body for sensing said information bearing elastic waves and converting them to an output electrical signal; said body having an attenuating section of radial thickness which is less than the radial thickness of other sections of said body for attenuating torsional elastic waves in non-zero order modes at frequencies below a preselected maximum frequency determined by the dimensions and configuration of said attenuating section, each of said transducing means comprising; a disc of ferro-electric material of a thickness corresponding to shear wave resonance at a predetermined operating frequency, having flat parallel end faces with a major portion of each half of said disc being electrically poled circumferentially in mutually opposing directions in diametrically opposing portions of the disc; a pair of electrodes bonded to one end face of said disc in registry with said major portions of said disc halves; and a unitary electrode bonded to and overlying the other end face of said disc.
 5. A torsional-wave delay line comprising: an elongate hollow cylindrical tube propagative of torsional-mode waves; a pair of circular discs, each having a thickness corresponding to shear wave resonance at a predeTermined frequency, of ferro-electrical material with each being electrically poled circumferentially in mutually opposing directions in diametrically opposed halves of the disc; first and second electrode pairs individually bonded to respective one end faces of said discs with the electrodes of each pair being disposed in registry with said diametrically opposing halves of said discs; first and second unitary electrodes individually bonded to and overlying the respective other end faces of said discs; and means for effectively bonding said discs individually to the respective opposite ends of said tube with said unitary electrodes individually engaging the respective ends of said tube.
 6. A torsional-wave delay line comprising: an elongated hollow cylindrical tube propagative of torsional elastic waves, said tube having a generally uniform cross-sectional configuration and radial wall thickness along a first section of its length and a second section having substantially uniform cross-sectional configuration and a radial thickness which is less than the radial thickness of said first section; a pair of circular discs, each having a thickness corresponding to shear wave resonance at a prdetermined frequency, of ferro-electric material with each being electrically poled circumferentially in mutually opposing directions in diametrically opposing halves of the disc; first and second electrode pairs individually bonded to respective one end faces of said discus with the electrodes of each pair symmetrically spaced on opposite sides of an imaginary diametrical line on their one end face; first and second unitary electrodes individually bonded to and overlying the respective other end faces of said discs; and means for effectively bonding said discs individually to the respective opposite ends of said tube with said unitary electrodes individually engaging the respective ends of said tube.
 7. A delay line as defined in claim 6 which further includes means for applying an electric signal in push-pull across the individual electrodes of one of said pairs. 